Tungsten recovery

ABSTRACT

The scarcity of high grade tungsten ores obliges users to seek to recover it from scrap materials, but drill bits in particular pose special problems. Existing proposals to use acidic hydrogen peroxide to oxidize and solubilize tungsten are extremely inefficient in reagent consumption per unit tungsten recovered. 
     According to the present invention hydrogen peroxide can be used considerably more efficiently with sub-stoichiometric reagent addition per stage, especially from 6 to 12 stages and by the use of a small amount of certain promoters which it is believed may form a water-soluble complex with tungsten which provides an alternative route for tungsten dissolution. Many of the promoters are aromatic hydroxyl or amino acids such as 2-hydroxybenzoic acid, 2-aminobenzoic acid, 4-hydroxybenzene-phenol and its related oxidation products such as 1,4-benzoquinone. Also, certain aliphatic acids such as tartaric and epoxy succinic acid show some promotional activity.

The present invention relates to improvements in a process for the recovery of tungsten and more particularly in improvements in a process employing acidic hydrogen peroxide to recover tungsten from scrap tungsten alloys.

Tungsten is a valuable and useful metal in for example electric light filaments and in drill bits, but high grade ores containing it are comparatively scarce so that there is continuing interest in its recovery from scrap materials. One source of scrap comprises tungsten alloys such as tungsten carbide pieces, each of which on average weigh several grams and are typically several mm such as at least 5 mm in size, many are 1 or 2 cm in size and some are even up to 10 cm in size. The tungsten carbide is often cemented with other metals such as cobalt, and can contain minor fractions of other metals such as titanium and tantalum typically often present as the respective carbides. The pieces can also be coated with a thin layer of, for example aluminium nitride or niobium or titanium or tantalum compounds. Both pyrometallurgical and hydrometallurgical methods have been suggested for recovering tungsten from scrap pieces, but the former methods currently suffer from the increased cost of energy in recent years, and the aforementioned coatings can interfere with tungsten recovery disproportionately. Other tungsten alloys that can be treated according to the present invention in piece form can contain significant proportions of copper, nickel and cobalt and minor amounts of group IVB and VB metals including titanium, tantalum, niobium and zirconium. A characteristic of tungsten alloy scrap pieces is that virtually none of the metal can be leached out using a mineral acid solution free from added oxidant.

Disintegration of drill-type tungsten/cobalt alloy scrap using a hydrochloric acid solution of hydrogen peroxide has already been suggested by Toyo Soda in Japanese patent application publication No. 1977-13408 which resulted in the leaching of the cobalt into solution and the disintegration of the resultant mass into a tungsten-containing powder. However, the problem still remains of recovering tungsten from the powder so that this disclosure represents only a partial solution to the problem of tungsten recovery.

In one other patent, Shwayder in U.S. Pat. No. 3,438,730, has advocated the use of phosphoric acid solution to disintegrate carbide masses, and once again he is producing a powder of tungsten carbide rather than extracting the tungsten into solution. In one comparison, he uses a mixture of hydrochloric acid and hydrogen peroxide, with at best 13.5% wt recovery. Even if that statement implied recovery into solution, which from the context is doubtful, it does not form the basis for a viable process for extracting tungsten from its scrap.

A different teaching is given in Japanese patent publication No. 50-21991 by Applied Science Research Institute Foundation, who suggest that tungsten itself can be recovered from tungsten scrap pieces by contacting them with an aqueous acidic solution of hydrogen peroxide having a certain range of compositions and under certain temperature conditions. The specification omits key details such as the relative amounts of solution and tungsten-lumps to use from the generalised description and from Examples of Accomplishment 1 to 3. In Examples of Accomplishment 4, the relative amounts of alloy and solution were stated, but both the concentration of hydrogen peroxide and the extent of dissolution of tungsten (as contrasted with the extent of recovery of tungsten from solution) are omitted. In consequence, the efficiency of use of the reagents can be determined only by repeat trials and can in no way be inferred from the specification itself. The credibility of the specification is further impaired by a significant internal inconsistency between the solution compositions stated generally in the text and that used in Example of Accomplishment 4, an inconsistency that has been resolved only by assuming that the compositions should be expressed as weight % rather than mol.%. There is naturally a presumption that if a document incorrectly states such a fundamental property as how much to use, little trust can be placed in any of its other assertions.

When the method of Japanese patent publication No. 50-21991 is applied to extracting tungsten from tungsten carbide-containing lumps in so far as the method can be ascertained, in repeat trials carried out prior to the making of the present invention, it was confirmed that some tungsten did indeed pass into solution, the extent being greater than in the absence of hydrogen peroxide. However, the extent of tungsten dissolution was poor and therefore very inefficient as regards H₂ O₂ utilisation even if the solution and lumps were kept in contact until all the substantial excess of hydrogen peroxide had been consumed. It was concluded that a substantial improvement in extent of tungsten dissolution would be needed before the method disclosed in principle in this patent publication could be regarded as industrially applicable. It was further concluded by comparison from both these and other trials that it was considerably more difficult to extract tungsten from tungsten alloy lumps such as drill bits than from fine powder obtained by grinding the lumps beforehand as evidenced by the comparative rates and extent of tungsten dissolution from the two forms of material. Moreover, it is usually possible to extract a substantial proportion of the other elements such as Co, Ni or Cu from powdered tungsten alloys, using a mineral acid solution free from added oxidant whereas this is not the case for tungsten alloy pieces. Accordingly, any assertions made concerning extraction from powders cannot be applied unquestioningly to lumps. Of course a successful improvement to the extraction from lumps of material would be beneficial by avoiding the stage of grinding an extremely tough material.

It is an object of the present invention to improve the efficiency of utilisation of the reagents in a process for recovering tungsten from tungsten alloy-containing pieces in which tungsten is dissolved into an aqueous acidic hydrogen peroxide solution.

According to one aspect of the present invention, there is provided a process for the recovery of tungsten from pieces of tungsten alloy-containing material in which the pieces are contacted with an aqueous acidic solution of hydrogen peroxide at a temperature maintained at from 10° C. to 50° C. until at least some tungsten has passed into solution, which process is characterised by contacting the pieces with the aqueous solution in a plurality of consecutive stages, each stage comprising the steps of contacting the pieces with a portion of the solution containing a substoichiometric amount of hydrogen peroxide maintaining contact until a substantial proportion of the free hydrogen peroxide content in the solution has been consumed and thereafter separating the tungsten-enriched solution from the solid residue.

By so doing, the amount of tungsten that can be extracted using a given amount of hydrogen peroxide and mineral acid under otherwise identical operating conditions can be increased beyond that obtainable in a single stage, the improvement increasing as the number of stages is increased. Whilst the major part of the benefit will usually be taken in the form of increased tungsten dissolution, some can, if desired, be taken in the form of a decreased overall amount of hydrogen peroxide or possibly mineral acid being employed. The division between the two ways of reaping the benefit is at the discretion of the user.

Although some gain is achieved using two stages instead of a single stage, it is advantageous to use at least 4 stages. In practice, it is often convenient to restrict the number of stages to no more than 25 stages and a substantial fraction of the gain can be achieved with no more than 15 stages. A balance between the increased overall duration of the multi-stage process and increased gain is often stuck by selecting a number of stages in the range of 6 to 12. It will be further recognised that where, for example 10 stages have been selected the user can terminate the process after a lesser number of stages such as 8 or 9, should he so desire and still retain a substantial proportion of the expected gain.

It is believed that tungsten is oxidised and then enters solution in the form of one or more aquated peroxide complexes. The precise form of the peroxidic species is not known, but it has been postulated that the eventually produced predominant species may have the formula (W₂ O₃ (O₂).2H₂ O)₂, which requires 51/2 moles H₂ O₂ per mole tungsten. Other species postulated include mineral acid residues within the complexes. Consequently, herein, for the avoidance of uncertainty, the term stoichiometric amount of hydrogen peroxide means 1 part by weight H₂ O₂ (as 100%) per part by weight of tungsten carbide pieces, or the corresponding 5.5:1 mole ratio for other tungsten alloys and corresponds to an H₂ O₂ Index value of 1.

The gain is maximised by maintaining contact between tungsten alloy pieces and aqueous solution until all free hydrogen peroxide has been consumed and thereafter separating solution from residue without delay or at least without inordinate delay, so as to minimise the likelihood of the peroxy species decomposing and thus lose tungsten from solution. Accordingly, it is preferable for the free hydrogen peroxide content of the solution to be monitored. By way of example, samples can be withdrawn at predetermined intervals either manually or automatically and analysed for free peroxide using a known titration technique employing potassium permanganate and a ferroin indicator or otherwise. Naturally, the frequency of the sampling can be higher or increased as the free hydrogen peroxide content approaches zero. Alternatively, after an initial assessment of the optimum contact period by rigorous monitoring, subsequent stages can employ the same period provided that they operate under the same conditions and the gain will be achieved nearly in full.

The proportion of solution used in each stage need not always be the same, and indeed may deliberately be different where it is desired to match contact time to a range of varying working periods, but under most circumstances it is most convenient for the portions to be substantially the same in each stage. By employing the same sized portions, each stage lasts approximately the same length of time and the operation is thereby simplified. It will be appreciated that the division into equal sized portions of the solution need not be viewed with mathematical precision but instead, some latitude can be tolerated.

In one method of operation, each batch of tungsten alloy scrap pieces is contacted in successive stages with portions of solution until a desired amount of tungsten has been extracted. In a preferred variation, the weight of scrap dissolved in each stage is replenished with fresh scrap before the next stage, so that in every stage the same weight of scrap contacts the selected proportion of solution, combining the advantages of stage-wise solution contact with steady state addition conditions.

Within the temperature range for carrying out the reaction, it is preferable to maintain a temperature of at least 25° C. and conveniently up to 40° C. This is readily obtained by cooling the aqueous solution which tends otherwise to increase in temperature as a result of the exothermic oxidation of decomposition reactions involving hydrogen peroxide. Convenient arrangements to effect cooling include pumping the solution in a loop through a heat exchanger or carrying out the reaction in an agitatedvessel equipped with a cooling jacket or cooling coils.

The mineral acid employed in the aqueous solution can be selected from hydrochloric, nitric or sulphuric acids. The improvement by employing the invention procedure is apparent using all three acids. In the event that it is subsequently desired to recover tungsten in solid form from the aqueous solution thereof it is preferable to select nitric acid solutions since they combine excellent tungsten dissolution with the possibility of scheelite precipitation subsequently. Use of sulphuric acid is recommended for use in processes in which the tungsten is to be recovered by boiling the liquor with precipitation of tungsten trioxide.

The concentration of mineral acid employed is more conveniently calculated as a function of the weight of the tungsten alloy scrap. As a generalisation, the extent of tungsten extracted into solution tends to increase up to a maximum as the relative amount of acid is increased until a point is reached at which any gain is minimal. That point varies in line with the proportion of other metals in the alloy and particularly cobalt, copper, nickel, tantalum and titanium. In addition, the amount of acid employed can depend also upon the nature of the scrap. Thus, in respect of tungsten carbide scrap, the amount of acid is often selected in the range of 0.2 to 3.0 parts by weight of nitric or sulphuric acid per part by weight of scrap, or from 0.05 to 0.4 parts of hydrochloric acid per part by weight of scrap. It is particularly suitable to use in total about 0.9 to 1.2 parts by weight of nitric or sulphuric acid per part by weight of tungsten carbide scrap when the other metals comprise about 7-15% w/w thereof. Where each stage uses approximately the same reagent quantities, this latter range converts, e.g. in the case of 8 stages to using about 0.11 to 0.15 parts by weight of acid per stage per part by weight of scrap. At 20-25% w/w other metal content the corresponding preferred amount is 1.8 to 2.2 parts by weight of sulphuric or nitric acids. The acid itself can be added in any conveniently available concentration, and preferably at least 1 molar. Nitric acid is often employed at 20 to 70% w/w concentration prior to its dilution with aqueous hydrogen peroxide. However, for non-carbide tungsten alloy scrap, the amount of acid used is often in the region of at least 0.2 parts by weight per part of scrap, and in many instances up to 1 part by weight, for any of the mineral acids.

Hydrogen peroxide is normally employed in an amount of, in total, at least 1.2 parts per part weight/weight of scrap, often up to 2.5 parts per part scrap and in many instances from 1.5 to 2 parts per part scrap. Amounts in excess of this may be used, if desired, but the benefit has not been found to match the increased reagent expenditure, since it is not readily practicable to recirculate any solution from which tungsten has been recovered whilst retaining any residual hydrogen peroxide.

Alternatively or additionally and in a second aspect of the present invention there is provided a process for the recovery of tungsten from pieces of tungsten alloy-containing material in which the pieces are contacted with an aqueous acidic solution of hydrogen peroxide at a temperature maintained at from 10° C. to 50° C. until at least some tungsten has passed into solution, which process is characterised by incorporating in the aqueous solution a promotional amount of a promotor selected from:

(a) ##STR1## in which A is an hydroxyl or amine substituent and B is selected from carboxylic acid, sulphonic acid and aldoxime groups and C an optional further substituent selected from alkyl, hydroxyl and amino groups

(b) phenol, amino phenols, 1,4-benzoquinone and dihydroquinone

(c) 1,3-dinitrobenzene acid and

(d) oxalic, expoxysuccinic and tartaric acids.

Whilst it will be recognised that many of the foregoing offer a pair of nitrogen or oxygen atoms that can chelate with the tungsten atom to form a complex, the mere existence of such a pair of suitably spaced N or O atoms is by no means sufficient as a means of distinguishing promoters from non-promoter additives. Thus, by way of example there are many compounds that are akin to the foregoing promoters and yet do not themselves promote, such as glycolic and citric acids. Moreover, long known transition metal chelating agents such as ethylene diamine tetraacetic acid and amino tri(methylene phosphonic acid) either show no promotional effect or actually impair the rate and/or extent of tungsten dissolution. This aspect of the instant invention is not predicated upon the following explanation which is given for illumination only. It is considered possible or even likely that at some stage during the extraction of tungsten into solution as a peroxy-complex, a complex is formed that does contain the promoter. The complex then further reacts in the case of certain compounds releasing the promoter to complex with fresh tungsten. However, for other compounds, no significant extent of complex formation occurs, and for yet others the complex formed is such that further reaction is inhibited. This explanation is consistent with the fact that some hydroxy carboxylic acids promote whereas others do not, and also the fact that some compounds known to stabilise hydrogen peroxide in acidic conditions, namely phenol, promote, whereas others, equally well known such as adipic acid if anything impair tungsten dissolution.

From the foregoing promoters it is especially desirable to select as promoter compounds from categories (a) and (b) and especially compounds from category (a) in which an acid group, be it carboxylic acid or sulphonic acid group is ortho or para to an hydroxyl or amino group. Within that sub-category are found salicylic acid (2-hydroxybenzoic acid), anthranilic acid (2-aminobenzoic acid), 4-hydroxy benzoic acid and 4-hydroxy benzene sulphonic acid, for the twin reasons that all are amongst the most effective promoters tested in the course of investigating the instant invention and advantageously do not form insoluble calcium salts that interfere significantly with subsequent processing of the tungsten solution to generate synthetic scheelite.

Most of the promoters are solid at use temperatures and they can be added in the form of granules or powders. The few others, namely the sulphonic acid compounds in category (a) and phenol in category (b) are liquid at 50° C. and can be added as such. All of the promoter can be introduced at the start of the tungsten extraction period or if desired it can be introduced progressively during the tungsten extraction period.

In the processes according to the present invention and when the promoter is introduced in the aforementioned forms, the amount of promoter to employ is usually at least 1 g per kilogram of tungsten alloy pieces and often at least 4 g per kilogram pieces. Extremely good results have been obtained in the region of 8 to 32 g/kilogram tungsten alloy pieces. Selection of optimal conditions in many instances occurs by balancing the extent of gain in tungsten recovery against the cost of any marginal increase or decrease in promoter: tungsten alloy pieces weight ratio. Amounts in excess of 32 g/kilogram can be used if desired, such as up to 50 g or even up to 100 g/kilogram of pieces. It is preferable for promoter to be introduced into every stage, should a multi-stage process be used. In many instances, the total amount of promoter added is from 80 to 160 g per kilogram of tungsten alloy pieces, when a multi-stage process in excess of 4 stages is employed.

Recovery for reuse of the promoter from pregnant tungsten solution presents practical difficulties so that it would be of economic benefit to minimise the amount of promoter consumed per unit of tungsten extracted. It has also been found that a saving in the consumption of any promoter that is solid at 50° C. can be made in a modification to the process without marked impairment in promotional effect by introducing it in the form of a fused body of material which presents a markedly lower ratio of surface area to volume, and maintaining in that form during the tungsten dissolution process. The resultant improved process is the subject of a contemporaneously filed application, entitled "Further Improvements in Tungsten Recovery". The fused block can be held in a porous container or affixed to some surface in the reaction vessel, most advantageously positioned so that the liquor washes over the block, but the agitator does not disintegrate the block. When such a fused block is used, the amount consumed seems substantially self-regulatory, the overall consumption generally falling in the range of 5 to 30 g promoter per kilogram alloy, i.e. often less than 20% of the amount that would be consumed if the same promoter had been introduced in powder or granular form. Thus, perhaps the most acceptable promoters are salicylic acid and anthranilic acid because they are not only solids but also amongst the best promoters.

The block of promoter is normally considerably larger than the pieces of tungsten alloy when it is first employed although, naturally as it is consumed it will grow progressively smaller. It is desirable to produce blocks weighing at least 30 g. A convenient weight of promoter to be present in the form of block or blocks, is often from 50 to 1200 g, and particularly 75 to 300 g per 1000 alloy pieces. Such amounts will permit a considerable weight of tungsten to be dissolved before the blocks need to be replaced or supplemented with fresh blocks.

The previously mentioned references with regard to multi-stage operation of the extraction process may be employed in conjunction with promoter addition to block form or as a powder, granules or liquid. One side effect of the promoter addition is that the benefit can be at least partially obtained by reducing the total amount of hydrogen peroxide added by an appreciable proportion, such as up to 15% of the amount in a promoter-free system. One way of achieving that is to reduce the number of stages used such as by one stage to achieve a given tungsten recovery, for example from 8 stages to 7 stages but otherwise employed the H₂ O₂ and acid concentrations that were applicable to the higher number of stages. This not only saves reagent costs but reduces manpower and other operational costs per unit of tungsten recovered. A further way of likewise saving hydrogen peroxide which is probably the most expensive reagent in the system, is to employ it at a lower weight ratio to alloy per stage whilst retaining the acid level at or similar to that which would be used without promoter.

It can be convenient, though not necessary, to subject the pieces to a pretreatment with a mineral acid solution which contains either nil or very little hydrogen peroxide, such as markedly less H₂ O₂ than would be introduced in any stage of for example an 8 stage process. The amount of H₂ O₂ in the pretreatment stage is thus usually up to 50 g H₂ O₂ per 1000 g tungsten alloy pieces. The acid concentration since it is present mainly to extract metals like cobalt is typically up to 150 g per 1000 g tungsten alloy pieces. Such an acid pretreatment can be especially desirable when the subsequent extraction process uses nitric acid.

Conditions for recovery of tungsten from a mineral acid solution are known in themselves. If the extraction process employed as an additive tartaric or oxalic acid, and/or sulphuric acid as the mineral acid it is necessary to recover tungsten solid as tungstic acid (WO₃) by concentrating the acid solution with precipitation of tungstic acid therefrom. Where no additive is added or where the additive did not interfere, it is preferable to recover the tungsten in solid form as synthetic scheelite by adding soda-alkali to the solution, thereby obtaining an insoluble cobalt oxide residue which is separated off and then adding soluble calcium to the cobalt-barren solution, with the result that insoluble calcium tungstate precipitates. The scheelite route advantageously also separates the titanium and tantalum to a very great extent from the tungsten. To effect similar separation of titanium and tantalum from tungstic acid it is necessary to redissolve the acid in soda or ammonia solution and thereafter precipitate scheelite.

The tungsten carbide pieces that can advantageously be used in the instant invention are typically sintered tungsten carbide pieces, which have been bound with cobalt and may contain a minor amount of tantalum and titanium and can also contain trace amounts of one or more other metals such as iron, niobium, zirconium, rubidium and cerium. The pieces can also be coated with aluminium nitride or tantalum or niobium or titanium compounds. Advantageously, such pieces can be used without being crushed or ground.

Conveniently, the present invention can employ concentrated hydrogen peroxide solutions commercially available, such as 35 to 70% w/w solutions stabilised with conventional amounts of sodium stannate and pyrophosphate and/or metal ion sequestering stabilisers such as polyphosphonic acids available for example under the Trade Mark DEQUEST 2000, 2010, 2040 or 2060. When the alloy contains a metal such as copper that is recognised as a strong decomposition catalyst for H202 in acidic solution, it is especially preferable to include a sequestrant or chelator for that metal so as to reduce its destructive impact. This may be effected by adding known sequestrants or by selecting as all or part of the promoter those compounds which sequester copper better, such as 4-hydroxybenzoic acid.

Having described the invention in general terms, specific embodiments will be given now in greater detail by way of example only.

In the subsequent Examples 1 to 42 and Comparisons A to P the tungsten carbide pieces had the following approximate analysis:

    ______________________________________                                                 Al.sub.2 O.sub.3                                                                      2.2%                                                                    SiO.sub.2                                                                            1.3                                                                      SO.sub.3                                                                             1.0                                                                      Cl    0.5                                                                      TiO.sub.2                                                                            2.8                                                                      Fe.sub.2 O.sub.3                                                                     0.2                                                                      CoO   5.8                                                                      Ta.sub.2 O.sub.5                                                                     3.4                                                                      WO.sub.3                                                                             80.6                                                                     ZrO.sub.2                                                                            0.5                                                                      Nb.sub.2 O.sub.3                                                                     0.8                                                                      CaO   0.1                                                                      Rb.sub.2 O                                                                           0.1                                                                      CeO.sub.2                                                                            0.1                                                              ______________________________________                                    

COMPARISON A AND EXAMPLES 1 TO 5

In this comparison and Examples, the difference between single stage and multi-stage extraction of tungsten was investigated. In each run, 100 g (approx) of tungsten carbide pieces each averaging about 10 g were contacted with agitation with an aqueous nitric acid solution containing in total 206 g hydrogen peroxide (calculated as 100% H₂ O₂) which equates to 11 moles H₂ O₂ per mole WC assuming the pieces to be entirely WC. The reaction vessel was water cooled to maintain a temperature throughout the extraction of 30°-35° C. The total weight of nitric acid employed in the solution is given in Table 1.

The concentration of H₂ O₂ in the extracting solution was monitored periodically by a standardised potassium permanganate titration on a small sample and when no residual H₂ O₂ was detected, the pregnant liquor was separated from any residual solids and analysed by X-ray fluorescence techniques, and by comparison the % tungsten extraction was calculated. The weight of solids was measured before and after the extraction as a check.

In the comparison, the entire amount of the extraction solution was contacted with the WC pieces at the same time in a single portion, whereas in the Examples an appropriate equal aliquot portion of the solution was contacted in each stage. The portion of solution used was freshly prepared each time in order that no loss of H₂ O₂ would occur by decomposition during storage prior to use.

                  TABLE 1                                                          ______________________________________                                                  Number of     Weight   % W                                                     Stages        acid (g) dissolved                                      ______________________________________                                         Comp A   1              82      24                                             Ex 1     2             147      45                                             Ex 2     4             147      56                                             Ex 3     8              73      49                                             Ex 4     8             147      72                                             Ex 5     8             220      73                                             ______________________________________                                    

From Table 1, it can be seen that both the number of stages and the amount of nitric acid present contributed to the overall effectiveness of the process. From interpolating a graph of the results of Examples 3 to 5 it can be seen that at 82 g nitric acid in 8 stages, the dissolution of tungsten would comprise about 57%. By comparison the actual measured figure in a single stage at 82 g nitric acid addition was only 24%. Examples 1, 2 and 4 demonstrate the progressive improvement in tungsten dissolution as the number of stages increases from 2 to 8 stages. Projection from those results suggests about 34% extraction at 147 g nitric acid in a single stage. Thus, increasing the stages from single stage to 8 stage contact, more than doubled the extent of tungsten dissolution using the same amount of H₂ O₂ and nitric acid.

Examples 3 to 5 show that benefit accrued from increasing nitric acid amount from 0.6 parts up to about 1.2 g parts per part w/w tungsten carbide pieces, but little more above that.

EXAMPLES 6 TO 12

These Examples were carried out in a similar manner to the preceding Examples and Comparison, but on a 400 g WC pieces scale. The WC pieces again averaged about 10 g each. The total amount of nitric acid used was always 1.2 g HNO₃ per g WC pieces, introduced in the form of a 70% w/w solution, and the total amount of hydrogen peroxide was always 2 g H₂ O₂ per g WC pieces. As before, the extraction temperature was maintained at 30°-35° C. by water cooling.

Each of these Examples employed as promoter, salicylic acid, added as a powder in the weight per stage shown in Table 2.

                  TABLE 2                                                          ______________________________________                                                         Weight of                                                      Number of       Promoter                                                       Stages          per stage (g)                                                                             % W Dissolved                                       ______________________________________                                         Ex 6    1           12         56                                              Ex 7    2           12         65                                              Ex 8    8           12         95                                              Ex 9    8           8          92                                              Ex 10   8           6          94                                              Ex 11   8           4          87                                              Ex 12   8           1          80                                              ______________________________________                                    

From comparing Table 2 with Table 1, it can be seen that salicylic acid was consistently acting as a promoter, even when added at only 1 g per stage. At 12 g per stage, the gain over a comparable promoter-free process amounted to about 20% tungsten irrespective of the number of stages, thus showing that the two features of staged extraction and promoter addition are cumulative. The amount of promoter added also affects the improvement in tungsten dissolution significantly up to about 6 g per stage but thereafter to a minimal extent.

EXAMPLES 13 TO 22 AND COMPARISONS B TO K

These Examples and Comparisons were carried out under exactly the same conditions and using the same starting materials as Example 10, but employing instead other promoters or known hydrogen peroxide stabilisers at 6 g per stage. The results are tabulated in Table 3, together with that from the additive-free Example 4 included as the bench mark.

In this Table, EDTA is ethylene diamine tetra acetic acid and NTMP is nitrilo trimethylene phosphonic acid (DEQUEST 2000 from Monsanto) both well known chelating agents and H₂ O₂ stabilisers.

                  TABLE 3                                                          ______________________________________                                         Additive                 % W dissolved                                         ______________________________________                                         Ex 4    --                   72                                                Comp B  EDTA                 73                                                Comp C  2-nitro phenol       73                                                Comp D  Benzoic acid         72                                                Comp E  Glycolic acid        71                                                Comp F  1,2-Ethanediol       70                                                Comp G  Phthalic acid        70                                                Comp H  Adipic acid          68                                                Comp I  Succinic acid        68                                                Comp J  Citric acid          67                                                Comp K  NTMP                 67                                                Ex 13   2-amino benzoic acid 97                                                Ex 14   4-hydroxy benzoic acid                                                                              95                                                Ex 15   4-hydroxy benzene sulphonic acid                                                                    95                                                Ex 16   2,5-dihydroxy benzoic acid                                                                          92                                                Ex 17   1,4-benzoquinone     90                                                Ex 18   Phenol               88                                                Ex 19   2-hydroxy benzaldehyde oxime                                                                        84                                                Ex 20   1,3-dinitro benzene  84                                                Ex 21   Tartaric acid        80                                                Ex 22   2-amino phenol       80                                                Ex 23   Oxalic acid          78                                                ______________________________________                                    

From Table 3, it can readily be seen that all of Examples 13 to 23 show promotion of tungsten dissolution. Most of the promoters were aromatic compounds substituted around the nucleus by an acidic group and either an amine or hydroxyl group. By contrast, with the exception of tartaric and oxalic acids, which were by no means the most effective, the aliphatic acids showed nil or even mildly demotional effects. Many of the aliphatic acids had been suggested as acidic H₂ O₂ stabilisers such as adipic acid EDTA, NTMP, so that this Table clearly proves that the improvement cannot be attributed to simple H₂ O₂ stabilistion. It will, however, be recognised that even though the additives in comparisons B to K do not enhance the tungsten extraction, the processes all adopt the other aspect of the invention, namely the use of a multi-stage extraction technique. Furthermore, the results prove that such additives can be included in the hydrogen peroxide if desired for some other reason, e.g. stabilising it in storage, without undue impairment in use.

EXAMPLE 24

In this Example, the conditions of Example 10 were repeated, but the promoter 2-hydroxy benzoic acid was introduced in the form of an immobilised, fused block on a glass rod instead of as a powder. The weight of the fused block was measured before and after each stage of the extraction in order to determine the amount consumed, and the tungsten extraction measured in the same way as before. The figure given for % tungsten extraction is the ratio of tunsten extracted over the amount that theoretically would be extracted if each stage extracted its aliquot share, i.e. one eighth in an eight stage process, expressed as a percentage. The same procedure was carried out using both fresh WC pieces and pieces that had already been subjected to a partial W extraction. Although the block contained over 50 g promoter, the amount consumed each stage varied within the range of 0.65 to 2.3 g per stage, whereas the tungsten extraction ranged from 98 to 107%, average 102% standard deviation 3%. There was some correlation between the higher consumptions and higher extractions within these ranges. From these results it can be seen that extraction of tungsten was much higher per stage than in the absence of the promoter.

EXAMPLES 25 TO 29

In these Examples, the procedure of Example 24 was adopted, but after each stage the weight of tungsten carbide process was restored to its starting amount, 400 g. In Examples 26 to 29 the amount of hydrogen peroxide introduced per stage was reduced as shown in Table 4. In each Example at least 4 stages were carried out and the results averaged. For purposes of comparison, the consumption of hydrogen peroxide per unit weight of tungsten extracted is shown, as H₂ O₂ Index--the lower it is, the more efficiently is H₂ O₂ being employed. It will be recognised that each Index unit represents a stoichiometric amount of H₂ O₂.

                  TABLE 4                                                          ______________________________________                                                  Weight (g)   Average                                                  Example  of H.sub.2 O.sub.2                                                                          Weight W   H.sub.2 O.sub.2                               No.      added        dissolved (g)                                                                             Index                                         ______________________________________                                         25       50           50.2       1.99                                          26       47.5         49.1       1.93                                          27       45           47.3       1.90                                          28       42.5         45.9       1.85                                          29       40           43.8       1.83                                          ______________________________________                                    

From Table 4, it can be seen that the actual amount of tungsten dissolved per stage falls as the amount of H₂ O₂ present per stage falls, but that its Index is becoming more favourable simultaneously, i.e. the same amount of H₂ O₂ in Example 29 dissolved more tungsten than in Example 25. To put these figures in their proper perspective, the index figure for Comparison A is 8.3.

EXAMPLES 30 TO 33

In these Examples, the mineral acid used was sulphuric acid at 1.2 g conc H₂ SO₄ (98% w/w) per g WC pieces instead of nitric acid, but otherwise the general procedure followed was as per Example 24 in Example 30 and with reduced H₂ O₂ additions per stage in Examples 31 to 33 as shown in Table 5. Thus, all these Examples employed a procedure in which 8 stages would theoretically extract all the tungsten and salicylic acid was present as promoter in the form of a fused immobile block. Examples 30 and 31 represent respectively averages of several sets each of 4 stages, each stage operating on the residue from the preceding stage whilst Examples 32 and 33 are results of a single set each of 4 stages. On average only 0.3 g promoter was consumed per stage, i.e. about 0.8% w/w of the tungsten carbide pieces.

The reaction time for each stage was found to be 8 hours for Example 30 and at least 9 hours for the other Examples.

                  TABLE 5                                                          ______________________________________                                                  Amount of    Amount W (g)                                                      H.sub.2 O.sub.2  (g)                                                                        dissolved (av)                                                                             H.sub.2 O.sub.2                              Example  per stage    per stage   Index                                        ______________________________________                                         30       50           57.1        1.75                                         31       40           55.2        1.45                                         32       37.5         53.0        1.42                                         33       35           45.0        1.56                                         ______________________________________                                    

From Table 5, it can be seen that an 8 stage process using a sulphuric acid/H₂ O₂ system in the presence of a promoter is extremely efficient at use of H₂ O₂. Thus, at best it was capable of extracting tungsten from pieces at a consumption of only 1.42 g H₂ O₂ per g tungsten extracted.

EXAMPLE 34

In this Example, Example 32 was repeated but in the absence of the promoter. Two sets of 4 stages each were carried out and the average tungsten dissolution was 45.4 g, giving an H₂ O₂ index of 1.65. By comparison with Example 32, this represents a loss of 7.6 g W dissolution per stage (1/6th to 1/7th) demonstrating that the promoter was still playing a significant role in the dissolution process.

EXAMPLES 35 AND 36

In these Examples the procedure of Examples 33 was repeated but using respectively double and quadruple amounts of acid and H₂ O₂ per stage, i.e. effectively employing a 4 stage or 2 stage process.

                  TABLE 6                                                          ______________________________________                                                  Amount of    Amount W (g)                                                      H.sub.2 O.sub.2 (g)                                                                         dissolved (av)                                                                             H.sub.2 O.sub.2                              Example  per stage    per stage   Index                                        ______________________________________                                         35        75          92.6        1.62                                         36       150          152         1.97                                         ______________________________________                                    

From Table 6, it can be seen that exactly as in the nitric acid system the efficiency of use of H₂ O₂ was best when the largest number of stages was used in the presence of the fused block promoter. The reduction in stages from 8 to 4 was similar in effect to the removal of promoter from the 8 stage system in terms of change in the H₂ O₂ Index.

EXAMPLES 37 AND 38

In these Examples the effect of increasing the number of stages in an HCl/H₂ O₂ system was briefly investigated in the same general manner as before. In each Example, the concentration of acid used in each stage was 110 mls of IN HCl per 100 g WC pieces. The Examples show stages in respectively 4 stage and 8 stage reaction with H₂ O₂, the additions per stage being respectively 20.5 g and 10.2 g H₂ O₂ (as 100%) per 100 g pieces. The reaction temperatures were maintained by water cooling to at or near the temperature shown in Table 7. Individual stage results are given, to demonstrate the similarity between them.

                  TABLE 7                                                          ______________________________________                                                                          % W                                           Example                                                                               Number of Stage    Temp   dissolved                                                                             H.sub.2 O.sub.2                        No     Stages    No       °C.                                                                            per stage                                                                             Index                                  ______________________________________                                         37     4         1        32      8.75  4.7                                                     2        32      8.25  5.0                                    38     8         1        35     6.9    3.0                                                     2        35     6.0    3.4                                                     3        35     5.9    3.5                                                     4        35     6.0    3.4                                    ______________________________________                                    

It will be recognised that Table 7 once again demonstrates that increasing the number of Stages improves the efficiency of use of H₂ O₂.

EXAMPLES 39 TO 42 AND COMPARISONS L TO P

Each of these Examples and Comparisons was conducted in the same manner as Example 38 i.e. 8 stages H₂ O₂ /acid reaction with the WC pieces at 35° C., but the amount of hydrogen peroxide used was increased per stage to 25 g per 100 g WC pieces and the acid level was 50 ml N HCl per 100 g WC pieces per stage. Where indicated in Table 8 promoters and other additives were introduced each at 2 g per 100 g WC pieces per stage.

                  TABLE 8                                                          ______________________________________                                                               % W                                                                            dissolved                                                                               H.sub.2 O.sub.2                                         Additive      per stage                                                                               Index                                           ______________________________________                                         Ex 39     --              7.1      3.5                                         Ex 40     Tartaric acid   9.1      2.7                                         Ex 41     Epoxysuccinic acid                                                                             8.6      2.9                                         Ex 42     2-hydroxybenzoic acid                                                                          9.1      2.7                                         Comp L    Benzoic acid    6.6      3.8                                         Comp M    Succinic acid   6.0      4.2                                         Comp N    1,2-Ethanediol  6.2      4.0                                         Comp P    Adipic acid     5.9      4.3                                         ______________________________________                                    

From Table 8, it can be seen that the effect of tartaric acid was somewhat greater in the HCl system than in the nitric acid system, and that once again an hydroxylated aromatic acid promoted tungsten dissolution.

EXAMPLES 43 to 51

In these Examples an alloy which had been subjected to an acid wash, to remove dirt and any iron coating was used, and which analysed as:

    ______________________________________                                         WO.sub.3           80.8%   (w/w)                                               CuO                15.2%                                                       NiO                3.7%                                                        Al.sub.2 O.sub.3   0.3%                                                        SiO.sub.2          0.1%                                                        ______________________________________                                    

The alloy was employed in pieces each weighing 20 g (approx) or more which were contacted with aqueous acidic hydrogen peroxide at about 25°/26° C. in 4 or 8 stages, each employing an aliquot portion which was separated from the depleted alloy pieces, when the hydrogen peroxide in the solution had been consumed, as determined in periodic tests for it on extracted small samples. In each trial, the total amount of H₂ O₂ employed was 1.7 parts (as 100% H₂ O₂) per part w/w tungsten alloy material. The trials were conducted with the residue of a previous stage being contacted with the fresh aliquot portion of hydrogen peroxide except for Ex. 47 and 48 in which material that had been removed in a previous stage was replaced with fresh material. The proportion of tungsten dissolved was determined as in previous examples by x-ray fluorescence techniques. In Examples 43 to 48, concentrated nitric acid was employed to provide an amount of approximately 0.28 ±/- 0.05 parts HNO₃ per part by weight tungsten alloy and in Examples 49 to 51 35% w/w hydrochloric acid was used to provide approximately 0.32 parts HCl per part by weight tungsten alloy. The promoter used was either 2-hydroxybenzoic acid (2HBA) or 4-hydroxybenzoic acid (4HBA) or a 50: 50 w/w mix of both (mix in Table 9). The promoters were employed in amounts of approximately 1 part per 40 parts by weight material.

The results are summarised in Table 9.

                  TABLE 9                                                          ______________________________________                                                   No.                      % W                                         Ex. No.   Stages  Acid      Promoter                                                                              dissolution                                 ______________________________________                                         43        4       HNO.sub.3 --     11                                          44        8       HNO.sub.3 --     18                                          45        4       HNO.sub.3 2HBA   30                                          46        8       HNO.sub.3 2HBA   34                                          47        8       HNO.sub.3 4HBA   56                                          48        8       HNO.sub.3 mix    60                                          49        4       HCl       2HBA   28                                          50        8       HCl       --     20                                          51        8       HCl       2HBA   48                                          ______________________________________                                    

From Table 9, it can be seen that once again the effect of increasing the number of stages leads to a marked increased in the extent of tungsten recovery both in the absence and presence of the promoter. Secondly, it can be seen that the addition of the invention promoter significantly improves tungsten recovery and thirdly that in the presence of copper it is advantageous to employ either as promoter or in conjunction therewith a compound that can effectively sequester or chelate with copper so as to reduce the extent of H₂ O₂ decomposition. 4HBA is better able to sequester copper than 2HBA and consequently when it is present, tungsten dissolution is improved.

EXAMPLES 52 TO 56

In these Examples, the tungsten alloy was an acid washed material that analysed as follows:

    ______________________________________                                         Metal (as oxide)                                                                               % w/w                                                          ______________________________________                                         WO.sub.3        88.1                                                           CuO             5.7                                                            NiO             1.8                                                            Al.sub.2 O.sub.3                                                                               0.9                                                            SiO.sub.2       0.7                                                            Fe.sub.2 O.sub.3                                                                               0.2                                                            ______________________________________                                    

The Examples were conducted in 4 or 8 stages contacting pieces of alloy each weighing about 20 g or more with aliquot portions of acidified aqueous hydrogen peroxide, to a total amount of 1.7 parts H₂ O₂ per part w/w of alloy. The total amount of acid used was about 0.3 parts HCl or about 0.5 parts HNO₃ per part w/w alloy. During each stage small samples are withdrawn and analysed for residual H₂ O₂ content. When no H₂ O₂ remained the residue and liquor were separated. The residue was contacted with a fresh aliquot portion except in Examples 55 and 56 where it was augmented to its original weight by addition of fresh alloy before being contacted with the next aliquot portion of liquor. The pregnant liquors were analysed by x-ray fluoroscence techniques to determine the amount of W extracted. In Examples 53 and 54 the amount of promoter was about 1 part per 12 parts w/w alloy and in Examples 55 and 56 about 1 part per 20 parts alloy, the abbreviations being the same as as for Examples 43 to 51.

                  TABLE 10                                                         ______________________________________                                                   No.                      % W                                         Ex. No.   Stages  Acid      Promoter                                                                              dissolution                                 ______________________________________                                         52        4       HCl       --     50                                          53        8       HCl       2HBA   63                                          54        8       HNO.sub.3 2HBA   68                                          55        8       HNO.sub.3 4HBA   85                                          56        8       HNO.sub.3 mix    86                                          ______________________________________                                    

From Table 10, it can again be seen that the use of the promoter and increasing the number of stages improves tungsten recovery and that in the presence of slightly less copper the extraction improves compared with the alloy of Examples 43 to 51. 

I claim:
 1. In a process for the recovery of tungsten from pieces of tungsten alloy-containing material of at least 5 mm in size in which the pieces are contacted with an aqueous mineral acid solution of hydrogen peroxide at a temperature maintained at from 10° C. to 50° C. until at least some tungsten has passed into solution, the improvement which comprises contacting the pieces with the aqueous mineral acid solution in a plurality of consecutive stages, each stage comprising the steps of contacting the pieces with a portion of the aqueous mineral acid solution containing a substoichiometric amount of hydrogen peroxide, maintaining contact until a substantial proportion of the free hydrogen peroxide content in the solution has been consumed, and thereafter separating the tungsten-enriched solution from the solid residue whereby the amount of tungsten that is extracted using a given amount of hydrogen peroxide and mineral acid under otherewise identical operating conditions is increased beyond that obtainable in a single stage.
 2. A process according to claim 1 wherein the number of stages if from 4 to
 25. 3. A process according to claim 2 wherein the number of stages is from 6 to
 12. 4. A process according to any preceding claim wherein approximately the same weight ratio of tungsten alloy to solution is employed in each stage by replenishing with fresh alloy before the next stage the amount of alloy removed in the preceding stage.
 5. In a process for the recovery of tungsten from pieces of tungsten alloy-containing material of at least 5 mm in size in which the pieces are contacted with an aqueous mineral acid solution of hydrogen peroxide at a temperature maintained at from 10° C. to 50° C. until at least some tungsten has passed into solution, the improvement which comprises incorporating in the aqueous solution a promoter selected from the group consisting of:(a) a compound having the formula: ##STR2## in which A is an hydroxyl or amine substituent, B is selected from carboxylic acid, sulphonic acid and an aldoxime group, and C is an optional further substituent selected from alkyl, hydroxyl and amino groups; (b) phenol; (c) amino phenols; (d) 1,4-benzoquinone; (e) dihydroquinone; (f) 1,3-dinitrobenzene acid; (g) epoxysuccinic acid; and (h) tartaric acid;the amount of said promoter being effective to increase the amount of tungsten that can be extracted using a given amount of hydrogen peroxide and mineral acid under otherwise identical operating conditions beyond that obtainable in the absence of said promoter.
 6. A process according to claim 5 wherein the amount of promoter employed per stage is at least 1 part per 1000 parts w/w of tungsten alloy pieces.
 7. A process according to claim 6 wherein the amount of promoter employed per stage is 8 to 32 parts per 1000 parts w/w of tungsten alloy pieces.
 8. A process according to claim 6 wherein the amount of promoter employed is 80 to 160 parts per 1000 parts w/w of tungsten alloy pieces in a process employing in excess of 4 stages.
 9. A process according to claim 5 wherein the promoter is selected from ortho or para hydroxy or amino substituted compounds in category (a) in which substituent B is a carboxylic acid or sulphonic acid substituent.
 10. A process according to claim 1 or 5 wherein the total amount of hydrogen peroxide introduced is from 1.5 to 2 parts w/w per part of tungsten alloy pieces.
 11. A process according to claim 1 or 5 wherein the tungsten alloy-containing material comprises tungsten carbide.
 12. A process according to claim 11 wherein the total amount of acid employed is from 0.5 to 3 parts w/w nitric or sulphuric acid or 0.05 to 0.4 parts w/w hydrochloric acid per part of tungsten carbide pieces.
 13. A process according to claim 12 wherein the total amount of acid employed is from 0.9 to 1.2 parts w/w nitric or sulphuric acid per part of tungsten carbide pieces.
 14. A process according to claim 1 or 5 wherein the tungsten alloy-containing material contains from 5 to 30% copper, nickel, cobalt, titanium, niobium and tantalum and the total amount of acid employed is at least 0.2 parts w/w of hydrochloric, nitric or sulphuric acid per part of alloy.
 15. A process according to claim 1 or 5 wherein the reaction temperature is maintaied in the range of 25° to 40° C.
 16. A process according to claim 1 wherein said aqueous mineral acid solution further comprises a promoter selected from the group consisting of:(a) a compound having the formula: ##STR3## in which A is an hydroxyl or amine substituent, B is selected from carboxylic acid, sulphonic acid and an aldoxime group, and C is an optional further substituent selected from alkyl, hydroxyl and amino groups; (b) phenol; (c) amino phenols; (d) 1,4-benzoquinone; (e) dihydroquinone; (f) 1,3-dinitrobenzene acid; (g) oxalic acid; (h) epoxysuccinic acid; and (i) tartaric acid. 